<!DOCTYPE html>
<html>

<head>
	<meta charset="utf-8">
	<title></title>
</head>
	<style type="text/css">
	body{
		display: flex;
	}
		.left{
			background-color: aquamarine;
			/* 剩余空间分布设置 */
			flex: 1;
		}
		.main{
			background-color: bisque;
			flex: 2;
		}
		.ad{
			width: 200px;
			height: 200px;
			background-color: skyblue;
		}
	
	</style>
<body>

</body>
		<div class="left">侧边栏</div>
		<div class="main">主要内容</div>
		<div class="ad">广告</div>
</html>